1. Field of the Invention
The present invention relates to a thermistor of which resistance is changed depending on temperature and a secondary battery to which the thermistor is attached, and more particularly to a combination structure of the thermistor and an object to which the thermistor is to be attached.
2. Description of the Related Art
A thermistor is a resistance element using a phenomenon that a specific resistance changes depending on temperature. The thermistor may employ NTC (Negative Temperature Coefficient) material whose resistance is decreased as temperature increases or PTC (Positive Temperature Coefficient) material whose resistance is increased as temperature increases. In particular, the PTC material shows a relatively low resistance at a normal temperature to allow electric current to flow through, while its resistance increases as much as 1,000 to 10,000 times of its origin state if a temperature of the material is increased due to an excess current or a surrounding temperature is increased, thereby interrupting an electric current. Thus, by using such a characteristic, the PTC material is commonly used for protecting various electronic parts from overheating or excess current. Such a thermistor is classified into a surface-mounted thermistor which is mounted on a surface of a substrate such as a printed board, and an attaching-type thermistor which is attached to an object such as a secondary battery.
Meanwhile, the secondary battery capable of being charged or discharged has low stability, namely it involves some risk of explosion resulting from abnormal states such as excess charging or excess discharging. Thus, a protection circuit or a PTC thermistor is commonly attached to the most secondary batteries in order to sense excess charging or excess discharging of a battery cell and then selectively isolating connection to an external circuit.
FIG. 1 shows a combination structure of a thermistor and a secondary battery to which the thermistor is to be attached. Referring to FIG. 1, a PTC thermistor 10 has upper and lower electrodes 12 and 14 on both sides of a PTC material layer 13. Upper and lower leads 11 and 15 are attached to the upper and lower electrodes 12 and 14, respectively. Meanwhile, a secondary battery cell can 30 to which the PTC thermistor 10 is to be attached is generally made of aluminum, and the lower lead 15 is generally made of nickel. Thus, in order to facilitate attachment between the lower lead 15 and the secondary battery cell can 30, which are made of different materials, a nickel-aluminum clad strip 20 is used. That is to say, the nickel-aluminum clad strip 20, configured so that a layer 21 to be combined with the lower lead 15 is made of nickel and a layer 22 to be combined with the secondary battery cell can 30 is made of aluminum, is inserted between the lower lead 15 and the secondary battery cell can 30. Then, the nickel layer 21 of the nickel-aluminum clad strip 20 is attached to the lower lead 15 by means of the spot welding 41, while the aluminum layer 22 is attached to the secondary battery cell can 30 by means of the supersonic welding 42.
The spot welding is a kind of resistance welding, which unites junction surfaces by pressing them at a high temperature or in a melted state by means of Joule heating around a welding spot. However, the spot welding requires too many and complicated process factors affecting on the welding, so it is not easy to obtain a high-quality junction. That is to say, the spot welding may give a junction of a desired quality only when three correlated process factors of a welding current, a dwell time and a pressing force should be controlled accurately. If any of the factors is not exactly controlled, it easily causes an inferior junction, like separation of a welded portion or easy detachment even by a weak impact.
In addition, in the combination structure of the thermistor and the secondary battery shown in FIG. 1, the spot welding 41 and the ultrasonic welding 42 are inevitably interposed, so a clearance on the surface of the welded portion is hardly ensured, thereby being apt to cause an inferior junction between the aluminum layer 22 of the nickel-aluminum clad strip 20 and the secondary battery cell can 30.